Use matchSelector and regression test for Firefox support.

[Closes #3]
Merge pull request #9 from ckundo/firefox-matcher-support

Use matchSelector and regression test for Firefox support.
Add MainRoleOnInappropriateElement audit rule, allow multiple ARIA roles
Add closure base.js and include in closure compiler command; update externs.js to work with latest closure compiler
Support for getting the URIs of all child iframes of a page.
Test case for multiple ARIA roles.
Minor cleanup.
Fix bug when attempting to parse data from postMessage(); add some logging spam
Store versions of iframe URLs with and without fragments
Store only fragment-less versions of URIs
Changes from upstream
Add MainRoleOnInappropriateElement audit rule, allow multiple ARIA roles
Add license header to BrowserUtils.js; remove references to JSON from externs.js
Remove ContentScriptFramework.js
Take opacity into account when calculating contrast ratio
Ignore all background images when calculating color contrast; check for url form background images in elementsWithMeaningfulBackgroundImage test; remove some log spam
Input type=button with value does not need a label
Fix indentation
Merge pull request #11 from dmazzoni-google/master

Input type=button with value does not need a label
Generate up to date axs_testing.js
Remove unused dependency
Add closure-compiler as a git submodule.

Developers need to initialize the submodules AND build the
closure-compiler project. Updated README accordingly.

[Finishes #1]